Radio telephones are used in vehicles and in other areas where there is often a high level of background noise. A high level of background noise can make it difficult for the user of the telephone to understand the speech being outputted by the loudspeaker or earphone of the telephone. The ability of the user to understand the speech being outputted by the loudspeaker is obviously essential and is referred to as the readability or intelligibility of the outputted speech.
In the past, the most common solution to overcoming background noise was to simply increase the volume at which the loudspeaker outputs speech. One problem with this solution is that the maximum output sound level that a loudspeaker can generate is limited. Due to the need to produce cost-competitive telephones, telephones often use lower-cost loudspeakers with limited power handling capabilities. The maximum sound level that such loudspeakers can generate is often insufficient due to high background noise.
Attempts to overcome the background noise by simply increasing the volume of the loudspeaker output can also result in overloading the loudspeaker. Overloading the loudspeaker introduces distortion to the loudspeaker output and further decreases the intelligibility of the outputted speech. Increasing the volume of the outputted speech also increases the problem of the microphone of the telephone picking up echoes from the loudspeaker output. The echoes of the speech output can result in the remote user of a linked, remote telephone hearing an echo of his or her own voice. This echo is annoying and can be removed with an adaptive echo cancellation filter or echo canceller. The complexity of the echo canceller should be minimized to limit the cost of the phone.
A telephone is needed that increases the intelligibility of speech outputted by the loudspeaker and that provides cost-effective echo cancellation.